11. Backends

Backends do the actual work of storing or retrieving data in response to LDAP requests. Backends may be compiled statically into slapd, or when module support is enabled, they may be dynamically loaded.

If your installation uses dynamic modules, you may need to add the relevant moduleload directives to the examples that follow. The name of the module for a backend is usually of the form:

back_<backend name>.la

So for example, if you need to load the hdb backend, you would configure

The hdb backend to slapd(8) is a backend for a normal slapd database. It uses the Oracle Berkeley DB (BDB) package to store data. It makes extensive use of indexing and caching (see the Tuning section) to speed data access.

hdb is a variant of the original bdb backend which was first written for use with BDB. hdb uses a hierarchical database layout which supports subtree renames. It is otherwise identical to the bdb behavior, and all the same configuration options apply.

Note: An hdb database needs a large idlcachesize for good search performance, typically three times the cachesize (entry cache size) or larger.

Note: The hdb backend has superseded the bdb backend, and both will soon be deprecated in favor of the new mdb backend. See below.

The LDAP backend to slapd(8) is not an actual database; instead it acts as a proxy to forward incoming requests to another LDAP server. While processing requests it will also chase referrals, so that referrals are fully processed instead of being returned to the slapd client.

Sessions that explicitly Bind to the back-ldap database always create their own private connection to the remote LDAP server. Anonymous sessions will share a single anonymous connection to the remote server. For sessions bound through other mechanisms, all sessions with the same DN will share the same connection. This connection pooling strategy can enhance the proxy's efficiency by reducing the overhead of repeatedly making/breaking multiple connections.

The ldap database can also act as an information service, i.e. the identity of locally authenticated clients is asserted to the remote server, possibly in some modified form. For this purpose, the proxy binds to the remote server with some administrative identity, and, if required, authorizes the asserted identity.

As previously mentioned, slapd-ldap(5) is used behind the scenes by many other Backends and Overlays. Some of them merely provide a few configuration directive themselves, but have available to the administrator the whole of the slapd-ldap(5) options.

For example, the Translucent Proxy, which retrieves entries from a remote LDAP server that can be partially overridden by the defined database, has only four specific translucent- directives, but can be configured using any of the normal slapd-ldap(5) options. See {[slapo-translucent(5)}} for details.

Other Overlays allow you to tag directives in front of a normal slapd-ldap(5) directive. For example, the slapo-chain(5) overlay does this:

"There are very few chain overlay specific directives; however, directives related to the instances of the ldap backend that may be implicitly instantiated by the overlay may assume a special meaning when used in conjunction with this overlay. They are described in slapd-ldap(5), and they also need to be prefixed by chain-."

You may have also seen the slapd-ldap(5) backend used and described in the Push BasedReplication section of the guide.

It should therefore be obvious that the slapd-ldap(5) backend is extremely flexible and heavily used throughout the OpenLDAP Suite.

The following is a very simple example, but already the power of the slapd-ldap(5) backend is seen by use of a uri list:

The URI list is space or comma-separated. Whenever the server that responds is not the first one in the list, the list is rearranged and the responsive server is moved to the head, so that it will be first contacted the next time a connection needs be created.

The LDIF backend to slapd(8) is a basic storage backend that stores entries in text files in LDIF format, and exploits the filesystem to create the tree structure of the database. It is intended as a cheap, low performance easy to use backend.

When using the cn=config dynamic configuration database with persistent storage, the configuration data is stored using this backend. See slapd-config(5) for more information

The mdb backend to slapd(8) is the recommended primary backend for a normal slapd database. It uses OpenLDAP's own Lightning Memory-Mapped Database (LMDB) library to store data and is intended to replace the Berkeley DB backends.

It supports indexing like the BDB backends, but it uses no caching and requires no tuning to deliver maximum search performance. Like hdb, it is also fully hierarchical and supports subtree renames in constant time.

In addition to the usual parameters that a minimal configuration requires, the mdb backend requires a maximum size to be set. This should be the largest that the database is ever anticipated to grow (in bytes). The filesystem must also provide enough free space to accommodate this size.

The meta backend to slapd(8) performs basic LDAP proxying with respect to a set of remote LDAP servers, called "targets". The information contained in these servers can be presented as belonging to a single Directory Information Tree (DIT).

A basic knowledge of the functionality of the slapd-ldap(5) backend is recommended. This backend has been designed as an enhancement of the ldap backend. The two backends share many features (actually they also share portions of code). While the ldap backend is intended to proxy operations directed to a single server, the meta backend is mainly intended for proxying of multiple servers and possibly naming context masquerading.

These features, although useful in many scenarios, may result in excessive overhead for some applications, so its use should be carefully considered.

The monitor backend to slapd(8) is not an actual database; if enabled, it is automatically generated and dynamically maintained by slapd with information about the running status of the daemon.

To inspect all monitor information, issue a subtree search with base cn=Monitor, requesting that attributes "+" and "*" are returned. The monitor backend produces mostly operational attributes, and LDAP only returns operational attributes that are explicitly requested. Requesting attribute "+" is an extension which requests all operational attributes.

The Perl backend to slapd(8) works by embedding a perl(1) interpreter into slapd(8). Any perl database section of the configuration file slapd.conf(5) must then specify what Perl module to use. Slapd then creates a new Perl object that handles all the requests for that particular instance of the backend.

The Shell backend to slapd(8) executes external programs to implement operations, and is designed to make it easy to tie an existing database to the slapd front-end. This backend is is primarily intended to be used in prototypes.

The primary purpose of this slapd(8) backend is to map a naming context defined in a database running in the same slapd(8) instance into a virtual naming context, with attributeType and objectClass manipulation, if required. It requires the rwm overlay.

The primary purpose of this slapd(8) backend is to PRESENT information stored in some RDBMS as an LDAP subtree without any programming (some SQL and maybe stored procedures can't be considered programming, anyway ;).

That is, for example, when you (some ISP) have account information you use in an RDBMS, and want to use modern solutions that expect such information in LDAP (to authenticate users, make email lookups etc.). Or you want to synchronize or distribute information between different sites/applications that use RDBMSes and/or LDAP. Or whatever else...

It is NOT designed as a general-purpose backend that uses RDBMS instead of BerkeleyDB (as the standard BDB backend does), though it can be used as such with several limitations. Please see LDAP vs RDBMS for discussion.

The idea is to use some meta-information to translate LDAP queries to SQL queries, leaving relational schema untouched, so that old applications can continue using it without any modifications. This allows SQL and LDAP applications to interoperate without replication, and exchange data as needed.

The SQL backend is designed to be tunable to virtually any relational schema without having to change source (through that meta-information mentioned). Also, it uses ODBC to connect to RDBMSes, and is highly configurable for SQL dialects RDBMSes may use, so it may be used for integration and distribution of data on different RDBMSes, OSes, hosts etc., in other words, in highly heterogeneous environments.

This backend has to be one of the most abused and complex backends there is. Therefore, we will go through a simple, small example that comes with the OpenLDAP source and can be found in servers/slapd/back-sql/rdbms_depend/README

We will presume you know how to create a database and user in PostgreSQL and how to set a password. Also, we'll presume you can populate the 'example' database you've just created with the following files, as found in servers/slapd/back-sql/rdbms_depend/pgsql